Thermal management is a critical issue in electronic parts such as the semiconductor lasers for optical communications and a high performance MPU. In most cases efficient removal of the heat generated by the semiconductor component is achieved by directly bonding a semiconductor component on a heat sink with a solder.
In the prior art an aluminum nitride (AIN) compact or a silicon carbide (SiC) compact has been the widely used material for the heat sinks above mentioned. In recent years, however, the thermal conductivities of these materials (250 W/mK for AIN and 270 W/mK for SiC) are getting insufficient to remove the generated heat from the electronic components because the more heat is being generated by the electronic parts as the output power of semiconductor lasers or the integration level of ICs is getting higher.
As for the thermal conductivity, diamond and cubic boron nitride (cBN) are the promising candidates as high performance heat sink materials. Heat sinks made of diamond or cBN are practically used for laser diodes for fiber-optic networks, which require extremely high reliability. This has been realized because the manufacturing technology of diamond has been so much improved either with CVD method or with high pressure and high temperature method that diamond has become commercially available. CBN compact which is made under high pressure and high temperature is also commercially available. cBN can also be produced by converting hBN (hexagonal boron nitride) being an allotrope of cBN at an ultra-high pressure and high temperature, followed by converting and sintering.
On the other hand, due to the progress in the optical communication technology, higher output power of semiconductor lasers is required. To meet this requirement, the semiconductor laser chips get larger in sizes and thus the thermal stress caused by the mismatch of the coefficient of thermal expansion (hereinafter referred to as CTE) between the semiconductor chips and the heat sinks gets a serious problem. More specifically, as the CTE of diamond is about 2.3×10−6/K and is much smaller than that of GaAs (5.9×10−6/K) or of InP (4.5×10−6/K), the reliability of the diode lasers is limited by the operation cycles as well as the soldering process.
To solve the above mentioned problems both an efficient cooling and a reduced mismatch of the CTE with semiconductor are required for a heat sink material. In other words, a material having both high thermal conductivity and reduced CTE mismatch with Si, GaAs, or InP is demanded.
Thus, the prior art discloses the use of a metal-diamond composite which has a relatively high thermal conductivity and also has a CTE which is substantially identical with that of semiconductor materials, as shown for example in JP-A-2-170452 (corresponding to U.S. Pat. Nos. 5,008,737 and 5,130,771), JP-A-4-259305 (corresponding to U.S. Pat. No. 5,045,972), JP-A-10-223812 and JP-A-11-067991.
JP-A-2-170452 discloses a composite material which is comprised of a metal matrix in which diamond particles have been thoroughly mixed and embedded wherein the proportions of diamond particles and metals are selected to form a composite having a CTE which is substantially the same as that of said semiconductor. The metal matrix of which is composed of at least one metal selected from the group consisting of copper, silver, gold and aluminum. Unfortunately, this composite material has some problems. One is the insufficient thermal conductivity because of the remaining voids within the composite caused by the poor wettability and reactivity between diamond and metals such as copper, silver, gold and aluminum. In addition to that because of the poor reactivity between diamond and the said metals, the surface roughness of the finished products is not smooth enough due to the removal of diamond particles from the surface during surface polishing process. The poor surface roughness of the heat sink makes the thermal contact between the semiconductor chip and the heat sink weak and thus the heat sink can not remove the heat efficiently enough.
JP-A-4-259305 discloses a composite material consisting of a metal matrix selected from the group consisting of aluminum, magnesium, copper, silver and alloys thereof and diamond particles. The disclosed production method of the composite material is as follows; (1) typical powder metallurgy techniques which include consolidation of the composite powder mixture by vacuum hot pressing, casting and explosive consolidation, (2) compaction of diamond powder and infiltrating the voids of the resulting compact with molten metal under pressure. Typical powder metallurgy techniques, however, have the same problems as those of JP-A-2-170452. With the simple method of compaction under high pressure and high temperature, which is used for making diamond compacts for cutting tools, diamond and copper cannot be sintered due to the influences of oxygen or nitrogen in the air.
JP-A-10-223812 and JP-A-11-067991 disclose the metal-diamond composite in which metal carbide is formed on the surface of diamond so as to improve the wettability between diamond and the above described metal and then the bonding strength at the interface between diamond and the metal is improved resulting in the improved properties of the composite. According to this method, however, the metal carbide at the interface degrades the thermal conductivity and a material of higher thermal conductivity cannot be obtained when compared with a material consisting of only diamond and copper.
In the above described technique as methods for the production of metal-diamond composites, methods comprising a sintering of mixed powder of diamond and metal under vacuum, hot press sintering, ultra high pressure sintering, etc. have been disclosed. Above all, ultra-high pressure and high temperature sintering method is most suitable for obtaining a substantially void free composite, at which the present invention aims. The ultra-high pressure and high temperature sintering method has been employed as a method of producing a sintered compact for a cutting tool material comprising diamond as a primary constituent and a ferrous metal such as cobalt as the binder. The ferrous metal works as a solvent of carbon and precipitate the carbon as diamond under ultra-high pressure and high temperature. Accordingly, the diamond particles bond strongly with each other resulting in that the CTE of the sintered compact is no higher than that of diamond. Moreover, the thermal conductivity of the compact is at most 400 W/mK by the adverse effect of the binder metal whose thermal conductivity is rather low.
JP-B-55-008447 and JP-B-56-014634 disclose the method of producing a diamond sintered compact whose binder is copper. The art disclosed by these patents is supplying melted copper or copper alloy through an orifice of a capsule filled with diamond powder placed in contact with the copper or copper alloy. The object of that invention is to provide a non-magnetic diamond compact by replacing a part of the binder metal with copper. By this method, however, one cannot obtain a diamond compact having thermal conductivity as high as 500 W/mK due to the small amount of oxidization of copper during the sintering process in which the capsule is partly broken under pressure and some leak of air occurs. Therefore, small amount of Cu2O and CuO exist in the sintered compact, as is shown in FIG. 1, the X-ray diffraction pattern of the sintered compact.
As above described, in the prior art, there have been no high performance heat sink materials suitable for high power semiconductor laser diodes or high performance MPUs due to the large mismatches of CTE between the conventional high thermal conductivity heat sink materials and semiconductor materials or due to the surface roughness of the polished surface. The principal object of the present invention is to provide a material whose thermal conductivity is higher than those of AIN or SiC (i.e. >500 W/mK) as well as whose CTE is close to those of the semiconductor chips made of InP or GaAs, i.e. 3.0-6.5×10−6/K.